1. Field of the Invention
The invention relates to an electric actuator system for hospital and care beds.
2. The Prior Art
The actuator system is according to the invention of the type which can be used for adjusting a hospital or care bed. In this type of bed the mattress is carried by a support surface having an adjustable backrest and legrest section, said support surface being mounted in a bed frame which may be raised and lowered by means of linear actuators in the actuator system. Further, the backrest and legrest sections of the bed may be adjusted by means of linear actuators. Normally, a type of linear actuator comprising a thrust rod, e.g. of the type described in WO 02/29284 A1 Linak A/S is used. This type of linear actuator comprises a spindle with a spindle nut. The spindle is driven by a reversible electric motor through a transmission. When the spindle is driven, the spindle nut is moved in an inwards or outwards direction depending on the direction of rotation of the electric motor. The linear actuator is a separate product with the spindle, transmission and electric motor enclosed in a housing. The housing typically consists of a motor housing and an outer tube. An inner tube is secured to the spindle nut. The inner tube is displaced in and out of the outer tube as the spindle nut is moved in and out on the spindle. In the opposite end of the spindle nut the inner tube comprises a front mounting. The outer side of the motor housing is furnished with a rear mounting. The front mounting and rear mounting are used to secure the linear actuator in the structure which should be adjusted.
For certain patients in the hospital and care sector it is necessary for the nursing staff to know whether the patient is in the process of leaving the bed or has left the bed. Such a bed is i.a. described in U.S. Pat. No. 4,934,468 Hill Rom Co. Inc. and U.S. Pat. No. 5,276,432 Stryker Corp. These hospital beds are equipped with a weighing system for weighing and/or monitoring the patient's weight. The weighing system can however also be configured to monitor the patient's position in the bed. The weighing system can further be connected to an alarm which can give off a signal in case the patient assumes a position where it is conceivable that the patient may leave the bed or has already left the bed. A bed having similar characteristics is described in EP 1 974 708 A1 Paramount. Here, changes in the patient's center of gravity are registered by a number of interconnected weight sensors located at each corner of the lying surface of the bed. By comparing the readings from each weight sensor, it can be detected whether a patient is sitting up and is thus potentially in the process of leaving the hospital bed, but naturally also whether the patient has left the bed.
Common for these types of bed structures is that they are intended for continuous weighing for accurate supervision of the patient's weight. In order to be able to do this with a sufficient accuracy high-end sensor with a high resolution are used. This fact is thus also reflected in the price of these bed structures, which are very expensive. The use of these beds is thus also limited to a select few patients requiring special treatment and special care.
A far simpler and cheaper construction is described in WO 2009/021513 A1 Linak A/S, which discloses an electromechanical linear actuator comprising the same elements as the electro mechanical linear actuator described in the preamble. Furthermore, this type of actuator comprises means for registering forces on the actuator. Means for registering these forces may be a load cell, such as a strain gauge or a piezo element. By registering the relative changes to the force on each actuator it can be determined whether a person is in the process of leaving a bed or has already left the bed. It is therefore the load of the person occupying the bed which is registered along with the position and position changes of the person in the bed. Although this construction as a whole constitutes a simpler solution than the solutions described above it is desired to provide an even simpler actuator system for a hospital or care bed which can detect whether a person is in the process of leaving the bed or has left the bed.